Diffusion of Macroparticulate Probes in Poly-(Acrylic)Acid: Water Solutions and Polyethylene Oxide Melt.

Abstract

Quasi-elastic light scattering spectroscopy was used to study the diffusion of various probe species dissolved in poly-(acrylic)acid:water solutions and in a polyethylene oxide melt. The diffusion coefficient for different size spheres (204 (ANGSTROM) to 1.5(mu)) was measured as a function of polymer concentration in polymer solutions of three different molecular weight (5(.)10('4), 3(.)10('5) and 1(.)10('6) amu). In the 3(.)10('5) solutions, the apparent hydrodynamic radius of the spheres increases markedly with increasing polymer concentration. At high polymer concentrations (C > 30 g/l), D and the viscosity were both described by functions of the form exp(-aC('(nu))); non-linear least-square fits gave (nu) (TURNEQ) 2/3 for spheres in the 3(.)10('5) and 1(.)10('6) solutions and (nu) (TURNEQ) 8/9 for spheres in the 5(.)10('4) solutions. With the smaller spheres (204 (ANGSTROM) and 800 (ANGSTROM)), polymer adsorption to a thickness of 100-150 (ANGSTROM) was apparent. In solutions of the 5(.)10('4) polymer, D followed the Stokes-Einstein equation. The Stokes-Einstein equation fails badly (by up to 10('4)) for spheres in high molecular weight polymer solutions. The decrease in the apparent hydrodynamic radii of larger spheres in high molecular weight solutions was ascribed to the shear thinning of a non-Newtonian polymer solution over the time and distance scales probed by spheres. The diffusion coefficient of 0.34(mu) and 0.31(mu) titanium dioxide particle in a poly-(ethylene oxide) melt (MW 7,500) as measured over the temperature range 85(DEGREES)C-160(DEGREES)C. By comparing the viscosity measured by a cone- and -plate viscometer with the microviscosity calculated through the Stokes-Einstein equation from the diffusion coefficients and hydrodynamic radii of the particles, the effective shear imposed by diffusing particles on the polymer melt was found to be in the range of 10('6)-10('8) sec('-1).